In sheet-fed offset printing machines, the sheets are stacked individually in a feeder and are removed by means of suction devices which are in phase with the printing machine. The sheets are then transported via a conveying table in a continuous shingled or fish scale stream to the positioning lays of a feed table. Alignment of the sheets takes place at the positioning lays in the sheet-conveying direction as well, as transversely thereto. At the same time, the position of the sheet is determined by sensory scanners, for example photoelectric sensors, which sense the leading edge of the sheet. As long as the sheets are positioned correctly, they are picked up by a pregripper and transferred to a first sheet-conveying cylinder.
Control devices for correct sheet running in the region of the positioning lays of sheet-fed offset printing machines are known, for example, from U.S. Pat. Nos. 5,096,183 (corresponding to German Pat. No. DE 4 001 120 A1) and 5,186,105 (corresponding to German Pat. No. DE 4 112 222 A1), and from U.K. Pat. No. GB 2 071 064 A (corresponding to German Pat. No. DE 3 044 643 A1). In these published devices, the control systems monitor the sheet feeding with the positioning lays during operation of the printing machine. More particularly, the sheet position is interrogated at the positioning line in one or more angular positions of a single-revolution shaft of the machine. However, these control devices do not address the problems associated with starting up the printing machine (i.e. the printing machine has no sheets present on the feeder table and a first sheet is conveyed to the-positioning lays (front lays) by switching on the feeder).
It is generally known that just after starting up, but also after a stoppage, a relatively large amount of waste paper is produced. This is due to two problems. The first problem concerns time constraints related to the vibrator-type inking unit of a sheet-fed offset printing machine. The inking unit has a relatively great inertia and requires time to build up a stable ink distribution. Until the stable ink distribution is maintained, the printing machine cannot properly process sheets. For example, in the case of very high vibrator cycles, the printing machine must carry out a multiplicity of revolutions before the vibrator transports ink from the ink duct roller to the first distributor roller.
The second problem is caused by the misalignment of the rollers with the inking unit. In virtually all sheet-fed offset printing machines, the vibrator is driven geared down (i.e. a movement of the vibrator between the duct roller, the first inking unit roller (distributor) and back to the ink duct roller takes place in more than one machine revolution). Vibrator cycles of 3:1 or 6:1 are usual, whereby one vibrator movement takes place in each case to 3 or 6 machines revolutions, respectively (single-axis shaft). In the case of a very low application of ink, however, it may also be necessary to set a vibrator cycle of 72:1.
In the case of multiple stoppages or production interruptions, the conditions for the re-inlet of a first sheet with respect to the rollers are arbitrary. It then depends on the skill of the printer by appropriate counter-control to reduce the amount of waste paper. However, depending on when the sheet inlet is cleared in relation to the position of the vibrator gearing, a compensation once carried out successfully may cause even more waste paper in a subsequent restart operation. This applies, in particular, to an automatic ink regulating algorithm for the purpose of avoiding start-up waste paper.